1. Field of the Invention
The present invention relates to a handover process for use in a CDMA (Code Division Multiple Access) mobile communication system implemented on an ALL-IP (Internet Protocol) network.
2. Description of the Related Art
In recent years, the popularity of mobile communication systems has been prompting communication service providers to reduce excessive burdens such as high frequency charges and communication facility expenses. In addition, as communication computerization advances, growing attention is directed to the fusion of mobile communication networks and IP networks, and may communication service providers are thinking about converting mobile communication networks into ALL-IP networks especially for the purpose of lowering facility costs.
At present, CDMA mobile communication systems which are employed as the main stream of third-generation mobile communication systems are capable of providing stable short-break (temporary blackout, intermittent discontinuity) free communication quality by allowing a mobile terminal to communicate simultaneously with a plurality of base stations by way of soft handover when the mobile terminal moves between cells. With the introduction of the IPv6 (Internet Protocol Version 6), the number of assignable IP addresses will drastically be increased, and an ALL-IP mobile communication network may be realized in the IPv6 environment with fixed IP addresses assigned to respective mobile terminals. However, sufficient efforts have not yet been made to study an IP technology for realizing soft handover between base stations on different IP networks.
For example, if a CDMA mobile communication system and an IP network are connected to each other, and IP communications are performed between a correspondent node (hereinafter referred to as CN) and a mobile node (hereinafter referred to as MN) using a fixed IP address without regard to movement of the MN, then a short-break or a packet loss tends to occur upon handover because it has heretofore been unable to connect the CN and the MN to each other via a plurality of base transceiver stations (hereinafter referred to as BTS) at the same time. As a result, the advantages of CDMA cannot fully be utilized, and communication failures are liable to occur. Therefore, the CDMA mobile communication system cannot enjoy the advantages of CDMA even if combined with the IP network technology.
Furthermore, inasmuch as the MN excessively issues binding update messages at a boundary between cells, and a large amount of transfer data is present between IP networks if the metric (an indicator of the number of hops and a bandwidth) from a home agent (hereinafter referred to as HA) to a link as a transfer destination is large, the overall network performance is possibly lowered.
The conventional problems will be described in specific detail below with reference to FIGS. 1, 2, and 3 of the accompanying drawings.
FIG. 1 is a view showing a system arrangement of a conventional CDMA mobile communication system. As shown in FIG. 1, a mobile communication system is connected to IP network 1, and CN 3 on link 2 connected to IP network 1 is communicating with MN 61 having a fixed IP address in cell 31 covered by BTS 21 on link 11. Communications between CN 3 and MN 61, including communications between BTS 21 and MN 61, are carried out according to the IP. When MN 61 moves toward cell 32, MN 61 attempts to make handover to BTS 22 covering cell 32.
After handover, since MN 61 communicates with BTS 22, MN 61 needs to communicate with CN 3 via link 12. However, since CN 3 does not recognize that MN 61 has moved to cell 32, CN 3 tries to communicate with MN 61 via link 11. Consequently, CN 3 loses communications with MN 61.
According to the mobile IP optimized for the mobile communication environment, MN 61 sends a binding update message indicative of handover to HA 41 on link 11. After MN 61 has made handover, HA 41 transfers data that is transmitted from CN 3 to link 11 to MN 61 which is now located in cell 32 covered by BTS 22 that is connected to link 12. Therefore, CN 3 and MN 61 keep communicating with each other without causing CN 3 to recognize that MN has moved to cell 32.
A handover process in the conventional CDMA mobile communication system will be described in specific detail below with reference to FIGS. 1, 2, and 3.
First, a procedure for MN 61 to make handover from cell 31 to cell 32 will be described below with reference to a sequence chart shown in FIG. 2.
(1) MN 61 performs IP communications with CN 3 via BTS 21 in cell 31 from which MN 61 is to make handover in step 201. BTS 21 converts between radio data and IP packets in step 202. This is indicated as state A in FIGS. 1 and 2.
(2) When MN 61 moves to a boundary between cell 31 and cell 32 in step 203, MN 61 makes handover from cell 31 to cell 32 as follows:
(3) MN 61 acquires a care-of address (hereinafter referred to as CoA) representing address information of link 12 in cell 32 to which MN 61 attempts to be connected, via BTS 22 in step 204.
(4) MN 61 includes the acquired CoA in a binding update message and sends the binding update message to HA 41 in step 205, and receives data from cell 12.
(5) When HA 41 receives the binding update message from MN 61, HA 41 encapsulates IP packet data for MN 61, and sends the encapsulated IP packet data via IP network 1 and a route 51 to BTS 22 which is indicated by the CoA contained in the binding update message in step 207.
(6) When BTS 22 receives the encapsulated IP packet data for MN 61 from HA 41, BTS 22 decapsulates the data, and sends the data as radio data to MN 61 in step 208. This is indicated as state B in FIGS. 1 and 2.
A procedure for MN 61 which has moved to cell 38 to make handover to cell 39 will be described below with reference to FIGS. 1 and 3. The handover operation is the same as when MN 61 makes handover from cell 31 to cell 32 as described above. The procedure will be described below in order to clarify that the data is necessarily transferred from HA 1 to a BTS in the cell to which MN 61 has moved.
(1) MN 61 performs IP communications with CN 3 via HA41 and BTS 28 in cell 38 from which MN 61 is to make handover in steps 209, 210, 211. BTS 28 converts between radio data and IP packets in step 211. This is indicated as state X in FIGS. 1 and 3.
(2) When MN 61 moves to a boundary between cell 38 and cell 39 in
(3) MN 61 acquires a CoA of link 19 in cell 32 to which MN 61 attempts to be connected, via BTS 29 in step 213.
(4) MN 61 includes the acquired CoA in a binding update message and sends the binding update message to HA 41 in step 214, and receives data from cell 39.
(5) When HA 41 receives the binding update message from MN 61, HA 41 encapsulates IP packet data for MN 61, and sends the encapsulated IP packet data via IP network 1 and a route 52 to BTS 29 which is indicated by the CoA contained in the binding update message in step 216.
(6) When BTS 29 receives the encapsulated IP packet data for MN 61 from HA 41, BTS 29 decapsulates the data, and sends the data as radio data to MN 61 in step 217. This is indicated as state Y in FIGS. 1 and 3.
As described above, the conventional CDMA mobile communication system allows CN 3 to continue communications with MN 61 without regard to the present position of MN 61 even when MN 61 moves through cells 31-39. However, the conventional handover process suffers the following drawbacks:
The first drawback is that when MN 61 moves to another link, a packet loss (loss of data) tends to occur. This is because CN 3 and MN 61 are unable to communicate with each other for a period of time after MN 61 cannot receive data from the former BTS when MN 61 moves to another link until MN 61 detects the CoA of the link to which MN 61 has moved, and also for a period of time until MN 61 tells HA 41 that MN 61 has moved and HA 41 starts to transfer the data. Japanese laid-open patent publication No. 2004-135178, for example, discloses a process of transferring data in advance from HA 41 to a cell (cell 32 in FIG. 1) near cell 31 from which MA 61 moves, and sending radio data to a nearby cell. Though the disclosed process can prevent a packet loss from occurring, it is disadvantageous in that radio resources cannot effectively be utilized because the radio data is transmitted to an unnecessary cell. Furthermore, if there are many nearby cells, the amount of data transferred by HA 41 is increased, tending to cause a reduction in the performance of nearby networks.
According to another process under study, before MN 61 makes handover, HA 41 buffers data from CN 3, and when MN 61 completes the handover to BTS 22 in cell 32, HA 41 starts to transfer the buffered data to MN 61 on link 21. This process does not produce a packet loss upon handover. However, the communications between MN 61 and CN 3 suffer a short-break until the handover from MN 61 is finished. In addition, it is assumed that HA 41 is liable to transmit a large amount of buffered data at one time, which flows into IP network 1, tending to lower the performance of the entire network.
The second drawback is that when MN 61 moves from link 11 to a link with a large metric, a delay may be caused during the transfer of data from HA 41 to MN 61 and the transfer of a large amount of data may cause a reduction in the performance of the entire network. This is indicated by the route 52 in FIG. 1.